1. Field of the Invention
The present invention is directed to an improved marine drilling riser of the type which utilizes means for providing buoyant support.
2. Description of the Prior Art
A substantial amount of exploratory drilling for deposits of crude oil and natural gas situated offshore is conducted from floating vessels. Such operations normally employ a marine riser which extends between the vessel and the subsea well. The riser is formed of a number of sections of pipe connected together end to end and serves to guide the drill string into the well and conduct drilling returns back to the vessel.
The riser must be supported to prevent its buckling due to its own weight, pressure differential caused by heavy drilling fluid, and forces acting on it as a result of waves, currents and the like. Generally, such support is provided by tensioning devices positioned on the vessel which apply an axial tensile force to the riser. Because the array of tensioning devices required for very deep water would be very cumbersome, tensioning has been supplemented with external buoyancy means affixed to the riser along the length thereof, permitting use of fewer tensioners aboard the vessel.
One method of providing external buoyancy is to affix cylindrical cans to the riser pipe. The cans are closed at the top and open at the bottom, allowing compressed air to be introduced into each can to expel the water therefrom and transform it into a buoyant member. The upper ends of the cans are rigidly affixed to the riser to support the can.
Failure from overstressing or metal fatigue is a severe problem in marine risers. Stress-inducing forces are exerted on the riser by heavy drilling fluid as well as by wind, waves and currents. Buoyancy can failures experienced with drilling risers employing buoyancy cans have led to the discovery that such cans have compounded the riser stress problem by substantially increasing stress in the riser.